1. Field of the Invention
The present invention relates to an optical scanner and an image forming apparatus provided with the same, which form an image with the light emitted from a light source on a surface to be scanned, such as a photoreceptor drum, while at the same time, scanning said light in a linear direction (main scanning direction).
2. Description of the Related Art
An electrophotographic image forming apparatus comprises an optical scanner for forming an image on a surface to be scanned (a surface of the photoreceptor drum) by scanning a light for writing an electrostatic latent image with a optical scanner unit such as a polygon mirror through a f-theta lens. In the thus constituted optical scanner, it is important to suffice the following three optical properties for a high-precision image formation (writing an electrostatic latent image).
In short, the first optical property is that the image curvature on the surface to be scanned is small (having uniform spot diameters). The second optical property is that the scanning speed of the light in the main scanning direction on the surface to be scanned is constant (having a good f-theta property). The third optical property is that the magnification (sub-scanning magnification) in the sub-scanning direction, which is orthogonal to the main scanning direction, is small. When the sub-scanning magnification is small, the spot diameters in the sub-scanning direction become uniform across the entire functional scanning area, even if the angles of the light reflecting surfaces of the optical scanner unit are not uniform. The third optical property is particularly important for a multi-beam scanning apparatus, that scans a plurality of light fluxes at one time, to keep the pitch distance of the scanning line uniform.
Furthermore, downsizing of optical scanners have been demanded to meet the space-saving needs for image forming apparatuses.
Conventionally, for example, Japanese Unexamined Patent Publication No. H07-318796 has been disclosing a f-theta lens system composed of the combination between a glass toric lens and a plastic toric lens, having a cylindrical lens on the incident surface side of the light flux and a toric surface on the light-emitting surface side.
In addition, in Japanese Unexamined Patent Publication No. H03-125111, an optical system comprising a holographic f-theta lens having a power only in the main scanning direction and a columnar lens having a power in the sub-scanning direction has been disclosed, wherein such as corrections of the f-theta property and the surface inclination, and the separation of a primary diffracted light from the diffracted lights in other degrees are conducted.
However, since the f-theta lens system disclosed in Japanese Unexamined Patent Publication No. H07-318796 is configured to have one cylindrical surface, there remains a problem in that the spherical aberration correction has a less degree of freedom, and thus the image curvature becomes large. Furthermore, the f-theta lens system disclosed in the embodiment 1 in Japanese Unexamined Patent Publication No. H07-318796 has a larger power in the main scanning direction of a glass toric lens disposed in the side of a surface to be scanned than a power in the main scanning direction of a plastic toric lens disposed in the polygon mirror side, and thus, the size in the main scanning direction of the glass toric lens is required to be relatively large, opposing the downsizing of the apparatus. Moreover, there still remains a problem in the f-theta lens system disclosed in the embodiment 2 in Japanese Unexamined Patent Publication No. H07-318796, in that since both the powers in the sub-scanning direction of the plastic toric lens and the power in the sub-scanning direction of the glass toric lenses are positive, the sub-scanning magnification becomes large when both the lenses are positioned close to a polygon mirror side for the purpose of downsizing the apparatus. If the sub-scanning magnification is large, the unevenness of the spot diameters of the light on the surface to be scanned relative to the unevenness of the reflecting surface of the optical scanner unit (such as polygon mirror) becomes large. Additionally, Japanese Unexamined Patent Publication No. H07-318796 does not particularly describe about the homogenization of the sub-scanning magnification.
In addition, in the optical system disclosed in Japanese Unexamined Patent Publication No. H03-125111, when the material of the columnar lens is, for example, glass, the lens has been more advantageous in cost performance than a f-theta lens combining a plurality of lenses. However, compared with a plastic lens, the glass lens has still been expensive. Moreover, in the optical system shown in the patent literature 2 (Japanese Unexamined Patent Publication No. H03-125111), if the material of the columnar lens is, for example, plastic, there remains a problem, in that the out-of-focus in the sub-scanning direction occurs from the change of a oscillation wavelength of the light flux and of the environment (particularly, change of temperature). And also, when the columnar lens is placed close to the surface to be scanned in order to avoid the occurrence of out-of-focus, the size of the columnar lens in the main scanning direction is required to be large, resulting in the system being against downsizing of the apparatus and expensive.
In view of the above problem residing in the prior arts, it is an object of the present invention to provide an optical scanner and an image forming apparatus provided with the same, wherein, with a compact apparatus configuration, the image curvature on the surface to be scanned, and the unevenness in the sub-scanning magnification as well as the optical scanning speed in the main scanning direction can be reduced, and moreover, the focus shift in the sub-scanning direction on the surface to be scanned is small with respect to the change in oscillation wavelength of the light flux and environment.